Adjustable constant current device



. P 1960 J. F. GRIMM 2,934,664

ADJUSTABLE CONSTANT CURRENT DEVICE Filed April 14, 1958 FIG! U i i VOLTAGE INVENTOR. JOHN E GRIMM ATTORNEYS ted S a s m Jolm F. Grimm, Newark, N.J., assignor to Tung-Sol Electric Inc., a corporation of Delaware Appncanen April 14, 1958, Serial No. 728,472

4 Claims. C1. s1s-s4 This invention relates to a circuit component which passes constant current when the applied voltage varies over a wide range of values. It is well suited to measuring circuits, especially those which are employed to sense and measure values of penetrating radiation. The device is useful in measuring equipment because its constant current value may be adjusted over a wide range to compensate for variations in the characteristics of other circuit components.

Constant currentdevices are well known in the electronic arts, one such component is the pentode vacuum tube which passes an anode current which varies only slightlygwith applied voltage. a device, however, re-- quires a heater current for its cathode, it is subject to many other sources of excitation and the anode current varies several percent from its average value. The photoelectric cell also may be regarded as a constant current component since the current through the tube under constant light excitation is substantially constant with variable anode-cathode voltage. This device has the disadvantage of requiring a steady luminous source which is often difiicult to install and maintain. The present device requires no external source of power, either electrical or optical, and it is not affected by changes in temperature nor by its period of use. The device to be described passes a constant current between applied voltages of 2.0 volts and 200 volts and the variation of current over this range is less than one-tenth of one percent.

One of the objects of this invention is to provide an improved adjustable constant current device which avoids one or more of the disadvantages and limitations of prior art arrangements.

Another object of the invention is to provide an adjustable constant current component which is small, easily handled, and will retain its characteristics for many years.

Another object of the invention is to provide an adjustable constant current device which is fully protected from external radiation and from shock and vibration.

Another object of the invention is to provide an adjustable constant current device which may be adjusted after being installed in a circuit to a precise current value.

The invention includes a small discharge space which contains a source of ionizing radiation such as radium or one of the radium compounds. An electrode is mounted within this discharge space and forms one of the terminals of the device. A larger adjustable volume enclosed in a metallic bellows is connected to the first discharge space by means of a small pin hole, the metallic bellows being adjustable in volume by means of a mechanical linkage which is under control of an operator. The device is operated over a pressure range which may be as low as one-quarter atmosphere and go as high as three atmospheres. The variation in pressure is accomplished entirely by varying the volume of the bellows.

For a better understanding of the present invention, together with other and further objects thereof, reference is made to the following description taken in connection with the accompanying drawings.

Patented Apr. 26, 1960 Fig. 1 is a side view of the constant current device.

Fig. 2 is a cross sectional view of the device shown in Fig. 1 taken along the longitudinal axis.

Fig. 3 is a graph showing the variations of current with applied voltage under different pressures.

Fig. 4 is a circuit showing use of the device in connectionwith measurement of penetrating radiation.

Referring now to Figs. 1 and 2, the constant current device is enclosed in two cylindrical containers, a small discharge tube 10 and a larger cylindrical container 11. These two containers are joined by welding and communicate with each other by means of a small hole 12. Tube 10 contains an electrode 13 which may be axially mounted to serve as a lead-in wire for external connection. Electrode 13 is mounted in an insulation bead 14. Tube 10 is preferably made of metal and therefore acts as the other electrode. On the inside of tube 10 a quantity of radioactive material 15 is deposited so that the radiations from this material will ionize the gas between the two electrodes and permit electrical current to pass. 7

Cylinder 11 houses a collapsible bellows 16 which may be made of metal or plastic. One end of the bellows is welded or soldered to the end of cylinder 11 about the opening 12 while the other end is secured to a threaded pinched off, thereby forming a metallic seal which may be reinforced by soldering over the end of the seal. Many gasses may be used in the discharge space as long as they are dry and are not subject to condensation within the range of temperatures encountered in use. For most applications the gas is under a pressure of slightly greater than one atmosphere with the bellows 16 fully extended but it will be obvious that the pressure of the gas is subject to a wide range of pressure values up to and including three atmospheres.

Disc 17 is threaded at its periphery and is engaged by an internal thread on a hollow cylindrical member 29 which is secured to an adjusting knob 21. The adjusting knob is held in place by a turned-over portion 22 formed as part of cylinder 11. Since the bellows 16 is stiff enough to resist torsional deformation, the disc 17 is not permitted to rotate, so when the knob 21 is turned, disc.17 is moved longitudinally along the axis of cylinder 11 and either compresses the bellows 16 and its confined gas or else expands the bellows with a subsequent expansion of the gas Within the bellows and the tube 10. By proper selection of the relative volumes within the tube 10' and the bellows 16, a ratio of pressures of three to one may be easily obtained by turning knob 21.

It will be noted that the construction of the device is such that the bellows 16 is completely enclosed by cylinder 11 and the adjusting mechanism. Electrical contact to the tube 10 may be made by a clamp holding the tube or by connecting a conductor toa conductive lug 23 secured to cylinder 11.

The radioactive material 15 within the tube 10 is preferably a compound of radium since radium produces constant ionization of the gas molecules within the envelope and therefore permits a constant current to pass between the electrodes even though the applied voltage may vary between wide limits. One such constant current component comprises a small metal cylinder which passes a current of 10- amperes at atmospheric pressure even though the applied voltage across its terminals varies between 20 and 400 volts.

Fig. 3 is a graph which shows the current-voltage characteristics of the device under different pressures. Curve A represents the current at atmospheric pressure while curves B, C, and D, represent the characteristics at higher pressures. When the voltage is raised above 400 volts the speed of the electrons within the tube becomes great enough to cause ionization of the gas molecules, therefore above this voltage the current increases rapidly.

Fig. 4 shows a simple scheme of connections foremploying the constant current device. A source of potential 25 is connected in series with a higher resistor 26 and the constant current device. The resistor 26 may have a value of 10 ohms and the source of potential may vary anywhere between 160 to 500 volts. Because of the constant current characteristics of tube 10, when set at 10 amperes, the voltage drop across resistor 26 will always be 100 volts. For purposes of measuring penetrating radiation a detector tube 27 may be shunted across the terminals of the constant current device. The detector tube 27 may be an ionization chamber or any other circuit bombarded by penetrating radiation. The additional current through detector 27 will be added to the current in resistor 26 and the additional drop may be detected by an electrostatic voltmeter 28 bridged across the resistor terminals. Such a circuit has been described and claimed in a patent application by John F. Grimm, filed February 10, 1958, Serial No. 714,128.

The foregoing disclosure and drawings are merely illusative of the principles of this invention and are not to be interpreted in a limiting sense. The only limitations are to be determined from the scope of the appended claims.

I claim: 7 1. An adjustable constant current device comprising, a

component which passes current when sealed envelope enclosing an ionizable gas at a pressure within the range of one-quarter to at least three atmos pheres, said envelope comprising two containers connected by an orifice for the transfer of gas, the first of said containers enclosing two electrodes defining a discharge space for the passage of electrical current which remains constant when the applied voltage across the electrodes varies within a range of 20 to 400 volts, a quantity of radioactive material adjoining the discharge space for ionizing the gas therein, the secondof said containers enclosed by a collapsible bellows, the volume of which is controlled by manual means.

2. An adjustable constant current device as set forth in claim 1 wherein said collapsible bellows is surrounded by a cylindrical guard and mechanical means coupled between the guard and the bellows controls the volume of the bellows.

3. An adjustable constant current device as set forth in claim 1 wherein said electrodes are insulated from each other and are connected to lead-in conductors for connection to an external circuit. 7

4. An adjustable constant current device as set forth in claim 1 wherein said radioactive material is deposited on one of said electrode surfaces and emits penetrating radiation into the space between the two electrodes.

Friedman Sept. 26, 1950- Brown Nov. 27, 1951 

